Documents or magnetic files, containing security sensitive information have to be sent throughout the world and it is often necessary to ensure that the documents have not been interfered with in transit or that if any attempt is made to reveal the information by an unauthorized person, an alarm is sounded or the information destroyed or erased.
Printed circuit boards in computers or data conversion equipment, often contain memory devices which should not be read (electronically), by anyone but authorized people. Any attempt to tap on to the board or device should ideally result in an erasure of all memory information.
The enclosure should provide protection from many forms of attack, from very crude destructive attacks to highly sophisticated laboratory attacks, but at the same time be sufficiently insensitive as not to be triggered by normal environmental effects such as temperature, humidity and vibration. Also in the case of printed circuit boards which must operate in the enclosure, the cable entry must not compromise the security of the enclosure and the enclosure must be sufficiently thermally conductive to stop the circuit from overheating.
A previous proposal is described in a paper entitled "Physical Security for the ABYSS System" by Steve H. Weingart, of I.B.M. in 1987 IEEE proceedings 49 89 4612 48B.
The system previously proposed uses very fine insulated wire which is wrapped around the device to be protected. The complete device is then potted in epoxy resin. The fine wire forms a long resistive element which is connected to a monitor circuit to detect changes in resistance.
However, this type of security enclosure has the following disadvantages:
1. No means of inserting the article to be protected after the enclosure has been fabricated.
2. In order to get high coverage, a lot of wire is used causing significant size and weight increase.
3. Corners are very difficult to cover. Ideal shape for this type of device is a sphere, but not many electronic circuits are this shape.
4. X-rays may be used to detect low coverage areas.
5. Epoxy can be scraped away to reveal elements which can be carefully tapped to create a hole.
6. Not practical for securing items which would be removed from the enclosure after a period of time.